In common rail fuel systems for diesel engines, fuel is delivered from an accumulator to each cylinder of an engine by a dedicated fuel injector. Each fuel injector comprises a valve needle that is moveable relative to a seating to permit or arrest the delivery of fuel. Movement of the valve needle towards and away from the seating is controlled by balancing opposed closing and opening forces. Fuel in a delivery chamber exerts an opening force on an end of the valve needle nearest the seating, which acts to lift the needle away from the seating. Fuel in a control chamber exerts a closing force on an end of the valve needle remote from the seating, which acts to force the needle against the seating.
The balance of the opening and closing forces is controlled by controlling fuel pressure in the control chamber. When the pressure in the control chamber is high, the closing force on the needle is sufficient to overcome the opening force exerted by the fuel in the delivery chamber, and the valve needle is retained in a closed position against the valve seating. When the fuel pressure in the control chamber is relatively low, the closing force is lower than the opening force, and the valve needle is lifted from the seating into an open position, such that fuel is injected into the engine cylinder.
FIG. 1 illustrates in cross-section a known control valve assembly 10 for use in controlling the fuel pressure in such a control chamber 12. A valve member 14 is moveable with respect to a housing 16 between a closed position and an open position by means of an actuator 18, such as a solenoid. In the closed position, the control chamber 12 is in communication with a high-pressure fuel supply 20. Fuel pressure in the control chamber 12 is therefore high. In the open position of the control valve 10, the control chamber 12 and the high-pressure fuel supply 20 are in communication with a fuel passage 22 that leads to a low-pressure drain (not shown). Pressure in flow passages 24 in the control valve and in the control chamber 12 is therefore relatively low. In this way, the control valve assembly 10 controls the pressure in the control chamber 12, which in turn controls the fuel injection.
In such control valves, the fuel in the flow passages 24 is prone to leakage, which leads to fuel loss, and hence leads to energy loss. This energy loss is undesirable because it decreases efficiency of the engine and results in increased CO2 emissions.
Leakage can occur during opening and/or closing of the valve 10, known as dynamic leak, and/or when the control valve 10 is in the closed position, known as static leak. Static leak is particularly significant as the control valve 10 is in the closed position for the majority of its lifetime. A major source of static leak is a clearance 26 between the valve member 14 and the surrounding housing 16. The clearance 26 must exist to allow for sliding movement of the valve member 14 with respect to the housing 16; however, the clearance 26 provides an undesirable pathway through which high-pressure fuel may leak out of the flow paths 22 of the control valve 10.
It will be appreciated that when the control valve 10 is in the closed position, the flow paths 22 within the control valve contain high-pressure fuel. The high operating pressures of today's fuel injectors mean that fuel in the flow paths 22 is typically at pressures of at least 2000 bar; high enough that fuel within the flow paths 22 exerts a pressure on the surrounding components that is sufficient to cause significant distortion.
Specifically, the high-pressure fuel in the flow paths 22 exerts an outward radial force on the housing 16, and an inward radial force on the valve member 14. The distortion forces the two components apart in the region of the flow paths 22. Away from the flow paths 22, the strain relaxes. However, the relaxation is gradual, and thus the housing 16 and the valve member 14 are still subject to distortion in the region of the clearance 26 between the valve member 14 and the housing 16. This distortion increases the size of the clearance 26 between the valve member 14 and the housing 16, and hence increases the tendency for leakage.
It is with a view to addressing the aforementioned disadvantage that the present invention provides an improved control valve assembly for a fuel injection system for an internal combustion engine.